Conventionally, with optical modulators that modulate light generated by a light source, a Mach-Zehnder interferometer may be used. In those optical modulators, a signal electrode and a ground electrode are provided along parallel optical waveguides. In recent years, because optical modulation methods are diversified, each optical modulator is often provided with two or more Mach-Zehnder interferometers. In those situations, by integrating the two or more Mach-Zehnder interferometers on one chip, it is possible to keep the size of the optical modulator small.
An optical modulator provided with two or more Mach-Zehnder interferometers is able to generate multi-level modulation signals by having a plurality of mutually-different electrical signals input thereto. In other words, by having the mutually-different electrical signals input from an external source to signal electrodes corresponding to the different Mach-Zehnder interferometers, the optical modulator is able to perform an optical modulation process that uses a multi-level modulation method such as a Differential Quadrature Phase Shift Keying (DQPSK) method.
In an electrical signal input section of an optical modulator, a connector may be provided; however, when one connector is provided for each of a plurality of electrical signals, the size of the optical modulator becomes large, and the mounting area increases. To cope with this situation, examples of methods for keeping the apparatus compact includes configuring the electrical signal input section by using a Flexible Printed Circuits (FPC) unit that has flexibility.
More specifically, the FPC unit has a plurality of wiring patterns corresponding to the plurality of signal electrodes of the optical modulator printed thereon, so that the electrical signals output from a driver are input to the optical modulator via the wiring patterns printed on the FPC unit. One end of the FPC unit positioned on the driver side is electrically connected to the driver by, for example, soldering the wiring patterns to electrodes that output the electrical signals supplied from the driver. The other end of the FPC unit positioned on the optical modulator side is electrically connected to the optical modulator by being inserted into a recessed section formed in the optical modulator and further having the wiring patterns soldered onto, for example, signal pins protruding downward from the ceiling face of the recessed section.
Patent Document 1: Japanese Laid-open Patent Publication No. 2007-123741
Patent Document 2: Japanese Laid-open Patent Publication No. 2014-029987
The electrical signals supplied from the driver to the optical modulator are signals having a relatively high frequency (e.g., approximately 30 GHz). When such signals having a high frequency (hereinafter “high frequency signals”) are transferred, it is known that, if a plurality of high frequency signal transfer paths are positioned close to each other, crosstalk may occur between the transfer paths. In other words, when the plurality of wiring patterns are printed on the FPC unit, if the wiring patterns are positioned close to each other, a problem arises where the crosstalk occurs, and characteristics of the electrical signals supplied to the optical modulator are thereby degraded.
To cope with this situation, one possible idea that can be used for reducing the occurrence of crosstalk is to enlarge the distance between the wiring patterns. However, since the FPC unit is used for the purpose of making the apparatus compact, it would not be desirable if the FPC unit were made larger as a result of enlarging the distance between the wiring patterns. Accordingly, it turns out that the plurality of wiring patterns are positioned apart from each other within the limit of the size of the FPC unit. It is therefore difficult to reduce the occurrence of crosstalk by a sufficient amount.
Further, another possible idea that can be used for reducing the occurrence of crosstalk is to form a ground pattern in the entire area of the FPC unit excluding the areas with the wiring patterns, so as to prevent electric fields from expanding from the wiring patterns. However, even if the ground pattern is formed in a large area, there is a limit to the effect achieved in reducing the occurrence of crosstalk. In addition, because the FPC unit is reinforced by the ground pattern, flexibility and pliability of the FPC unit are impaired. As explained above, it is difficult to reduce the occurrence of crosstalk by a sufficient amount, by simply enlarging the distance between the wiring patterns or providing the ground pattern having a large area.